It is known from German Pat. No. 813,839 issued Sept. 17, 1951 to R. H. Hobrock to continuously manufacture steel tubing by pulling a solder-clad steel strip off a supply continuously, shaping it into a multilayer tube, and fusing the solder to hold the tube together. This fusing can either be done by inductively heating the multilayer tube to generate eddy currents therein, or by passing an electric voltage through the tube so as to heat and fuse the solder. This method is intended to produce a relatively seamless tubing of considerable strength. What is more, the method works at relatively low cost and can operate continuously.
The principal difficulty with this method is it is almost impossible to ensure uniform quality of the tubing being produced. Frequently the solder is insufficiently fused so that a cold-solder joint is produced, or at other times the solder is melted at such a high temperature that it diffuses into the metal it is supposed to be bonding to or runs off altogether, forming no connection at all.
Even in a wholly magnetic method, such as described in German published patent application No. 2,828,960 filed June 28, 1978 by H. U. Dietzel and W. Muffke, it has normally been found impossible to achieve uniform results with respect to the solder layer between adjacent layers of the tube.
Accordingly recourse has been had to a method of the type described in German published application 2,839,684 filed Sept. 8, 1978 by H. Abel, W. Muffke, and H. U. Dietzel. In accordance with this method sections of the multilayer tubes are cut as they are produced continuously and then are treated in a batch-type operation in a heating chamber so that uniform heating can be guaranteed, along with a good solder joint.
As the result of the complexity of achieving uniformly good results with the known methods, the production of multilayer tubing in accordance with the above-described procedures has been very small. Normally unreliable quality of such tubing has led to little or no use of the patented method.